Electrostatic coating apparatus



7, 1968 R. 1.. LA FAVE ETA!- ELECTROSTATIC COATING APPARATUS 2Sheets-Sheet 1 Filed Oct. 26, 1966 INVENTOR.

RICHARD L. LA FAVE NORBERT M. ZUPAN Aug. 27, 1968 R. L. LA FAVE ETAL3,398,892

ELECTROSTAT IC COAT ING APPARATUS 2 Sheets-Sheet 2 Filed Oct. 26. 1966INVENTOR.

RICHARD L. LA FAVE NORBERT M. ZUPAN United States 3,393,892ELECTROSaTATHI COATTNG APPARATUS Richard L. La Fave and Norbert M.Zupan, Indianapolis,

Ind., assignors to Ranshurg Electra-Coating Corp Indianapolis, Ind, acorporation of Indiana Filed Oct. 26, 1966, Ser. No. 589,749 9 Claims.(Cl. 239-15) ABSTRACT OF THE DTSCLGSURE An atomizer for use in anelectrostatic spray coating system comprises an elongated body having aflow-guiding surface which receives liquid coating material from anelongated slot and guides it for fiow as a film to a discharge edge fromwhich it is electrostatically atomized. The slot is supplied from anelongated feed chamber containing longitudinally adjustable closureswhich regulate the effective length of the slot. The atomizer mayinclude means for preliminarily wetting the slot walls and flowguidingsurface with a solvent to promote the formation of the liquid materialinto a uniform film.

This invention relates to electrostatic spray coating apparatus of thegeneral type shown in US. Letters Patent No. 2,893,893 to W. W. Crouse,wherein the coating material flows as a thin film to an extended edge ofa charged atomizer and is electrostatically atomized from such edge. Theinvention is primarily concerned with features which contribute to theuniformity of distribution of the liquid coating material along theatomizer-edge and with features which permit a straight-edged atomizerto project sprays having widths that can be varied up to the fulleffective length of the atomizer.

A preferred form of apparatus embodying the invention comprises anelongated body having a downwardly inclined surface leading to ahorizontally extending discharge edge. The upper edge of such surface isdefined by one wall of a longitudinal slot communicating with a feedchamber which extends from one end of the body to the other, parallel tothe discharge edge. A second and similarly extending chamber, ormanifold, communicates with the feed chamber through a plurality oflongitudinally spaced ports each of which is controlled by a valve.Within the feed chamber there are disposed two plugs or closures whichcan be adjusted along the chamber and the spacing between whichdetermines the effective length of the chamber and slot and thereby thewidth of the spray discharged from the atomizer. Extending along theatomizer in rear of the aforesaid slot is a pipe provided with a seriesof longitudinally spaced discharge openings adapted to discharge asolvent onto the aforesaid downwardly inclined surface. An insulatedsupport is provided for the atomizer, such support being connected tothe atomizer by means which permits the atomizer-edge to be accuratelyleveled and the atomizer as a whole to be rotated about a horizontalaxis to vary the inclination of the downwardly sloping surface.

When the atomizer body, positioned with its discharge edge spaced from agrounded article to be coated, is connected to a high-voltage source andthe manifold is connected to a source of liquid coating material, thecoating material will flow into the manifold and thence through openedports into the feed chamber between the plugs therein, the coatingmaterial then emerges from the feed chamber through the slot and fiowsdownwardly as an exposed film over the downwardly inclined surface tothe discharge edge, where it will be electrostatically atomized by theelectrostatic field existing between the atomizer and the article. Thecharged coating-material par- 3,398,892 Patented Aug. 27, 1968 ticlesthus formed will be deposited on the article by the electrostatic forcesof the field.

Other features of the invention will become apparent from the followingmore detailed description and from the accompanying drawings, in which:

FIG. 1 is a view, partially diagrammatical in character, of a completespraying apparatus showing the atomizer in end elevation;

FIG. 2 is a rear elevation of the atomizer and its supporting means;

FIG. 3 is a fragmental rear elevation of the atomizer on an enlargedscale, certain parts being broken away to appear in cross-section;

FIG. 4 is a vertical section on the line 44 of FIG. 3;

FIG. 5 is a lan view showing a device suitable for use in locating andsecuring in place the plugs which determine the effective length of thefeed chamber; and

FIG. 6 is an end elevation of the atomizer body showing a preferredmethod of forming the paint-emitting slot.

In FIGS. 1 and 2 we have shown an elongated, horizontally disposedatomizer, designated in its entirety by the reference numeral 10,arranged to discharge electrostatically atomized liquid coating materialon to the vertically moving stretch of a continuous web 11. The atomizer10 shown comprises a body 12 of metal or other conductive materialconnected through a lead 13 to the ungrounded terminal of a high-voltagesource 14, the other terminal of which is grounded. Liquid coatingmaterial from a source 15 delivered to the interior of the atomizer body12 through a conduit 16, preferably by means of a variable speed,positive-displacement pump 17. Coating material from within the atomizeremerges, in the manner hereinafter described, along the upper edge of aplane film-supporting face 18 provided on the body 12, such face slopingforwardly and downwardly to a discharge edge 1?. The web 11 iseffectively grounded, with the result that there exists between it andcoating material on the discharge edge 19 an electrostatic field whichserves to atomize finely divided, electrically charged particles fromsuch material and to deposit such particles on the web 11.

An insulating support for the atomizer 10 may comprise a plurality oftubular posts 21 within each of which an adjusting screw 22 is rotatablymounted in fixed axial location. Each of the screws 22 carries a nut 23projecting outwardly through a vertical slot 24 in the wall of the postand is prevented from rotating by engagement with the sides of suchslot. At the location of each post 21 the atomizer body 12 is providedon its rear face with a U- shaped bracket 25 which receives a tongue 26rigid with the associated nut 23 and pivotally connected to said bracketby a horizontal pivot pin 27. Preferably, the several pins 27 arerigidly secured in their respective tongues 26 and the brackets 25 areprovided with set-screws 28 (FIG. 3) which, when tightened against thepins, hold the atomizer in fixed position about the common axis of thepins. Each of the screws 22 projects upwardly beyond the end of its post21, where it is provided with a knob 29 by which it can be rotated tocontrol the vertical position of the associated nut 23. The number ofposts 21 may vary, depending on the length of the atomizer.

The construction of the atomizer 10 is best illustrated in FIGS. 3 and4. As there shown, the atomizer body 12 is formed of a long metal bodyof generally trapezoidal cross-section having two parallel bores 31 and32 extending longitudinally through it from end to end. The bore 31,hereinafter referred to as the feed chamber, is located between thedischarge edge 19 and the other bore 32, hereinafter referred to as themanifold. The two bores intercomrnunicate through a series of ports 33which are spaced apart longitudinally of the body 12 and each of whichis provided with an independently operated shutoff valve 34screw-threadedly mounted in a boss 35 on the rear face of the body andprovided at its rear end with a knob 36 by which it can be rotated toopen or close its associated port 33. The ends of the manifold 32 areclosed by pipe plugs 37 at least one of which is provided with anappropriate fitting 38 through which the manifold is connected to thecoating-material supply conduit 16.

Extending along and defining the upper edge of the face 18 on theatomizer body 12 is the front wall of a slot 40 through which coatingmaterial emerges from the chamber 31 to flow by gravity as a thin filmover the face 18 to the discharge edge 19. The effective length of thechamber 31 and slot 40, and hence the width of the liquid film on theface 18, is controlled by two plugs arranged for longitudinal adjustmentin the chamber. As such plugs may be identical in construction, it willbe necessary to describe but one of them, shown at the left in FIG. 3.As there shown, each plug comprises a hollow cylindrical body 41 havinga sliding fit in the chamber 31 and provided at its inner end with arabbet groove receiving an O-ring seal 42. A screw 43 mounted in thebody 41 has a head which engages the seal 42 to compress it into sealingengagement with the wall of the chamber 31. Exteriorly, the plug body 41has a longitudinal groove in which is fixed a baffle 44, preferably ofTeflon or like material, that occludes the slot 40. Desirably, the innerend of said baifiie extends over, and is notched to receive, the seal 42to prevent the seal from being distorted into the slot when the screw 43is tightened.

For the purpose of positioning each of the plugs in the chamber 31andoperating the screw 43, we may use a tool of the type shown in FIG.5. Such tool comprises a long rod 46 having at one end a knob 47 and atthe other end a bit 48 adapted to be received in a slot 49 in the end ofthe shank of the screw 43 and having a width greater than the diameterof the rod. At its outer end, the plugbody 41 has an internal flange 51provided with a central opening 52 (FIG. 4) of a diameter to pass therod 46 and having two diametrically opposite grooves through which thebit 48 may pass when properly oriented. To change the position of aplug, the tool of FIG. is inserted, bit-end first, into the chamber 31from the end of the atomizer-body 12 until the bit 48 engages the end ofthe plug. The tool is then rotated to bring the bit into the plane ofthe grooves of opening 51 and moved inwardly, rotating it again ifnecessary, to bring the bit into the screw-slot 49, whereupon the screwmay be loosened to relieve the O-ring seal and free the plug formovement along the chamber 31. Inward movement of the plug is effectedby pressure applied to the screw 43 through the tool, outward movementby pulling the tool outwardly while the bit 48 is oriented to engage theinner surface of the flange 51. The inner and outer ends of the opening52 may be chamfered to facilitate passage of the bit 48. When the plugis remote from the end of a long atomizer body, insertion of the bitthrough the opening 52 can be facilitated by the use of a bit-centeringpilot 53 slidable on the rod 46 and adapted to be secured at any desiredposition therealong by a set-screw 54.

In an electrostatic atomizer, the quality of atomization obtained can beaffected by the rate of liquid supply, increasing rates of supplytending to coarsen the atomization. In stationary atomizers, such asthat herein shown, the character of atomization required for theproduction of even relatively poor finishes cannot be obtained if therate of coating-material supply is much in excess of about cc. perminute per inch of effective discharge edge. Where the slot iscoextensive with the discharge edge and of limited extent in thedirection of flow through it, it is not practical, at such a low rate ofsupply, to control that rate as a function of slot width. This is sobecause, even at extremely low hydraulic pressures, a slot limiting flowtherefore employ a slot 40 which is too wide to have any flow-regulatingefiect and control the rate of coating-material supply by regulating thespeed of the pump 17. In such an arrangement, the front edge of the slotserves in effect as a weir, and in order to provide uniform distributionof the coating material along the discharge edge 19 of the atomizer itis therefore necessary that such slotedge be accurately straight. Forslot-widths of the range we find it convenient to use, it has been foundimpractical to form the slot as a saw-kerf with a thin milling cutter,as the inherent flexibility of such a cutter causes the slot to departfrom accurate straightness, while tooth-marks on the slot walls wouldresult in undesirable local irregularities. We therefore prefer to formonly the front wall of the slot in the atomizer-body 12 and to employ asa rear wall the opposed, narrow front face of a bar or cover 55 whichextends longitudinally of the body 12 and is secured thereto by screws59. A preferred manner of forming the front slot-wall, shown in FIG. 6,involves the use of a rigid milling cutter 56, one edge portion of whichcuts through the material of the atomizer body into the chamber 31creating a generally V-shaped, longitudinal groove having two plane sidesurfaces 57 and 58 which are at a substantial angle to each other andboth of which are parallel to the axis of chamber 31. The surface 57,which constitutes the front wall of the slot 40, lies in a plane atleast generally radial of the chamber 31. Its intersection with the face18 forms the weir over which the coating material spills to flow downthat face to the discharge edge 19. The surface 58 lies in a planespaced from the axis of the chamber 31 by a distance slightly less thanthe radius of such chamber. The cutter 56 is indicated in FIG. 6 as acylindrical cutter, providing a angle between the surfaces 57 and 58;but if a different angular disposition of those surfaces is desired, afrusto-conical cutter can be used. The milling operation performed bythe cutter 56 leaves a sharp edge at the intersection of the surface 58with the wall of the chamber 31 and provides a gap between that edge andthe surface 57. The width of that gap should be somewhat greater thanthe intended width of the slot 40 in order to prevent contact of thesharp edge with the baffles 44; but otherwise the gap should be asnarrow as practicable in order that it will not inhibit the sealingaction of the O-ring 42.

We have found that when the atomizer as so far described is first placedin operation, the coating material tends to emerge from the slot 40 onlyat more or less isolated locations spaced alon the slot and to flow downthe face 18 in separated films rather than as a continuous sheet andthat material later emerging tends to follow the same paths. We havefound that the persistence of such a condition can be prevented bytemporarily reducing the viscosity of the liquid on the face 18, aresult readily accomplished by supplying to coating material emergingfrom the slot and to unwetted portions of the face 18 a solvent orthinner which is compatible with the coating material. For that purpose,the atomizer may comprise a longitudinally extending tube 63 disposed inrear of the slot 49, connected to a solvent-source 64 through a valvedconduit 65 (FIG. 3), and provided with a series of longitudinally spacedopenings 66 directed to discharge solvent generally toward the slot andthe film-supporting face 18 of the atomizer body.

For the purpose of emptying the bores 31 and 32 after completion of apainting operation, the atomizer body 12 may be provided at the ends ofsuch bores with drain openings, conveniently screw-threaded openingsreceiving removable closure screws 60.

In Setting up the apparatus for a coating operation, the atomizer 10 isadjusted to the desired position about the axis of the pins 27 andsecured in such position by tightening the screws 28. If the spray fromthe atomizer is to be projected generally horizontally, as is the casein the arrangement shown in FIG. 1, the face 18 of the atomizer body 12is desirably disposed to slope downwardly and forwardly to the dischargeedge 19 at an angle to the horizontal of about 45 or somewhat less; butin any event, the atomizer should be so disposed that the lower surfaceof the body 12 has at least a slight downward slope toward the edge 19in order to prevent the coating material from flowing around the edgeand rearwardly on such lower surface. If the spray is to be projected ina general downward direction, the slope of the face 18 may be increasedeven to a point where the face is vertical. By adjustment of the screws22 in the posts 21, the atomizer may be brought to the desired verticalposition and its discharge edge 19 leveled. The plugs which serve as endclosures for and determine the effective length of the feed chamber 31are set to provide a spray of the desired width and location along theatomizer, the valves 34 between the plugs are all opened, and the valves34 outwardly beyond the plugs are all closed.

With the atomizer so positioned, the pump 17 is placed in operation at aspeed such as to supply coating material at the desired rate; and ascoating material begins to emerge from the slot 40 solvent is suppliedto the tube 63, and the supply of solvent is continued until the coatingmaterial has spread on the face 18 into a film which is continuousbetween the bafiies 44, whereupon the supply of solvent may beterminated. Thereafter, with the atomizer charged, coating materialsupplied to it emerges through the slot 40 between the bafiles 44, flowsdownwardly as a thin film across the wetted face 18 of the atomizerbody, and is electrostatically atomized and deposited as described aboveand in the aforesaid Crouse patent.

An accurate indication as to the horizontal disposition of the dischargeedge 19 is provided by the appearance of the coating material beingatomized at such edge. The electrostatic field at such edge distorts thecoating-material film into a series of spaced cusps the tips of whichrepeatedly break off to form the charged spray particles. Assumingsubstantial uniformity of field strength along the edge 19, theappearance at one portion of such edge of cusps having a wider spacingand larger diameter than the other cusps indicates that such edgeportion is too low, a condition which can be corrected by appropriateadjustment of one or more of the screws 22.

The number of posts 21 used to support the atomizer will depend upon thelength, rigidity, and inherent straightness of the atomizer body. Thethree posts shown in FIG. 2 were found advisable for an atomizer 52" inlength designed to produce a spray having a width of 48" at thedischarge edge. As shown in FIG. 2, the posts 21 are supported fromtheir lower ends. However, remov- =ability of the pivot pins 27 permitsthe posts to be inverted and supported from overhead without invertingthe atomizer.

While we have shown an atomizer as arranged to spray coating materialgenerally horizontally on to a vertically moving, continuous web, itwill be apparent that, by swinging the atomizer on the pins 27, it canbe arranged to spray downwardly on to a web which is moving in a generalhorizontal path. It will also be apparent that the continuous web can bereplaced by a conveyorized series of discrete articles. The path andform of the spray projected from the discharge edge 19 will bedetermined largely by the pattern of the lines of force of the fieldexisting between that edge and the article or articles being coated, andit is therefore possible, for example, for the atomizer, oriented as inFIG. 1, to spray downwardly on to an article or articles passing belowit. However, it is usually desirable that the cross-section of theatomizer body point generally in the direction of spray projection, asin that manner the spray will be more compact and the spray particleswill have shorter distances to travel before being deposited.

We claim as our invention:

1. An electrostatic atomizer for use in an electrostatic spray coatingsystem, said atomizer comprising an elongated body having a plane faceterminating in an extended discharge edge from which liquid coatingmaterial is to be electrostatically atomized, said body being providedwith an elongated feed chamber and, communicating with said chamber, anelongated slot through which liquid'coating material from said chambercan emerge to flow as a thin film over said face to the discharge edge,a closure for each end of said feed chamber and slot, at least one ofsaid closures being adjustable longitudinally of the body to establishthe effective length of the chamber and slot, and means for conveyingliquid coating material into said chamber between said closures.

2. An electrostatic atomizer according to claim 1 wherein said lastnamed means comprises an elongated passage extending parallel to saidchamber and communicating therewith through a plurality oflongitudinally spaced ports, each of said ports having an associatedvalve by means of which it can be opened or closed.

3. An electrostatic atomizer according to claim 1 wherein eachadjustable closure comprises a generally cylindrical member freelyslidable in said chamber and having a radially projecting bafilereceived in said slot to occlude it, an elastic annular seal carried bysaid body and distortable by axial compression into sealing engagementwith the wall of said chamber, and releasable means on said cylindricalmember for so distorting the seal.

4. An electrostatic atomizer according to claim 3 wherein said bafileoverlies said sealing member to prevent its distortion into the slot.

5. An electrostatic atomizer according to claim 3 wherein saidseal-distorting means comprises -a screw mounted in said member andhaving an axially disposed threaded shank provided at its end with aslot for the re ception of a tool-bit by which the screw can be rotatedto release or distort the seal, said member having a central passagethrough which the tool-bit can be inserted into engagement with saidslot, said member also having in said passage an abutment engageable bythe tool-bit upon withdrawing movement thereof.

6. An electrostatic atomizer according to claim 1, wherein both saidclosures are adjustable longitudinally of said body.

7. An electrostatic atomizer for use in an electrostatic spray coatingsystem, said atomizer comprising an elongated body having a plane facebounded on one side by a discharge edge from which liquid coatingmaterial is to be electrostatically atomized, said body being providedwith an elongated cylindrical feed chamber extending parallel to saiddischarge edge and with a longitudinal external groove having first andsecond side surfaces disposed at a substantial angle to each other andintersecting the wall of the feed chamber along parallel lines defininga narrow gap, said first side surface intersecting the plane face ofsaid body on a line parallel to said discharge edge, and a coverremovably secured to said body and overlying the second side surface ofsaid groove, said cover having a side extending parallel and in spacedrelation to said first groove surface and defining therewith anelongated slot through which coating material supplied to said feedchamber can emerge to flow over said bodyface as a thin film to saiddischarge edge.

8. An electrostatic atomizer according to claim 7 wherein saidfeed-chamber is provided with a longitudinally adjustable end closurehaving a baffie projecting into said slot.

9. In combination with an electrostatic atomizer for use in anelectrostatic spray coating system, said atomizer having a flow-guidingface terminating in an extended discharge edge from which liquid coatingmaterial is to be atomized, said atomizer also having, spaced from saiddischarge edge, an elongated slot through which liquid coating materialsupplied to the atomizer emerges to flow as a thin film over said faceto said discharge edge, means mounted on said atomizer for discharging aliquid References Cited UNITED STATES PATENTS 7/1920 Diebold 239-13 8 av Miller 239-15 Ransburg et a1 239-15 Crouse 239-15 Heuschkel 239-15Marvin 239-15 EVERETT W. KIRBY, Primary Examiner.

